1. Field of the Invention
This invention relates to linear actuators and, in particular, linear actuators that employ shape memory alloy (SMA) elements to provide their motive power.
2. Description of Related Art
A new category of linear actuators employing SMA wire was introduced in U.S. Pat. No. 6,326,707, issued Dec. 4, 2001. These linear motors achieve useful displacement with significant force through the use of a Displacement Multiplied SMA mechanism. It is expected that SMA actuators will gain widespread acceptance and use in the near future, due to the fact that they produce much higher output force relative to their weight than current technologies (largely motors and solenoids). Their compact size allows them to fit into much smaller envelopes than existing actuators, solving numerous ‘real-estate’ and engineering issues. SMA actuators are long-lasting, easily performing a hundred thousand cycles. They can be manufactured simply, and in large quantities, inexpensively.
Due to the fact that the SMA motive elements (generally wires) are activated by thermal cycling, these devices are inherently sensitive to ambient temperatures, and susceptible to spontaneous actuation when the ambient temperature exceeds the SMA transition temperature. Nitinol wire is available commercially in formulations that have phase transition temperatures of 70° C. (LT) and 90° C. (HT). Thus if the ambient temperature exceeds these phase transition temperatures, the device will actuate inadvertently, with unpredictable and perhaps unfortunate consequences.
The specifications for many products and mechanical assemblies have ambient temperature tolerances that may exceed the phase transition temperatures of commonly available shape memory materials. For example, automobile manufacturers have operation and safety margins that most often require survivability, and even operability, in the temperature range of 100° C. to 120° C., which is greater than the transition temperature of Nitinol wires known in the prior art. There is an unmet need in the prior art for SMA actuators that can operate normally (intentionally actuated by powered operation) yet are prevented from operating spontaneously when the ambient temperature exceeds the SMA phase transition temperature. This need exists even if SMA materials are improved to transition at higher temperatures, for there will always be some uses for SMA devices that push the temperature limits of the materials.